This invention relates to a two-level (binary) display device such as a liquid crystal, electroluminescense or plasma display device which makes a display oh two levels H (High) and L (Low), and more particularly to such a display device provided with a control circuit for displaying video color signals.
Since liquid crystal, electroluminescense and plasma display devices are all similar in that they all make a display in two levels, a liquid crystal display device is used to explain the background of the present invention. With reference to FIG. 10 which is a prior art control circuit for a liquid crystal display which receives binary-converted TTL-level signals such as separate red (R), green (G) and blue (B) video signals from a video output port of a personal computer, for example, an OR gate takes a logical sum from an RGB signal to produce a video signal V.sub.D and this video signal V.sub.D is extracted under control of a memory control circuit 1 only during display periods according to the timing of horizontal and vertical synchronizing signals H and V and received by a memory device 2. Data received by the memory device 2 are retrieved by a liquid crystal display control circuit 3 and supplied through a liquid crystal driver circuit 4 to a liquid crystal display panel 5 at a timing necessary for the liquid crystal display. In a circuit thus structured, the memory device 2 and the memory control circuit 1 are used for matching the high-speed video data with the low-speed liquid crystal display and the memory control circuit 1 serves to generate many kinds of input and output timing signals for this purpose.
When an RGB color signal is received by a prior art display device, however, colors cannot be distinguished because the display device is for making a display only in two levels and also because a logical sum is taken from the RGB signal to generate the video signal to be inputted into the memory device 2. Suppose, for example, that an input signal for red, blue and green stripes has been received on a cathode ray tube screen of a personal computer. If it is attempted to display it on a liquid crystal display panel, however, the entire surface makes an ON display and the viewer cannot tell whether it is a display of an all white input signal.